1. Field of the Invention
The present invention relates generally to breathing ventilators, and more particularly concerns the control of the exhalation valve during the breathing cycle.
2. Description of the Related Art
Lung ventilation systems provide artificial respiration to patients whose breathing ability is impaired. Typically, such systems are capable of operating in any of several modes, selectable as a function of the degree of breathing assistance prescribed for a particular patient. At one extreme, the ventilator is given complete control including when each breath is delivered and the volume of gases delivered during each breathing cycle. At the other extreme, the ventilator permits "spontaneous" breathing wherein the inspiration and expiration phases are commenced in response to efforts by the patient. Varying degrees of control within these two extremes can be taken advantage of.
System pressure, both during the inspiration phase as well as the expiration phase is controlled by the exhalation valve. During inspiration, the exhalation valve in effect serves as a pressure relief valve to limit to a preselected value the maximum pressure the patient is exposed to. Upon exhaling, the breath is expelled through the exhalation valve, which during the expiration phase attempts to maintain pressure at a lower preselected second value.
An important parameter which is controlled by the ventilator during spontaneous as well as fully automated modes of ventilation is the residual pressure against which the patient exhales. It has been found that by maintaining a slight positive pressure, the collapse of alveoli, the bronchial passages and possibly the entire lung in severely compromised patients, can be prevented. As a patient regains strength, the positive end expiratory pressure (PEEP) is gradually reduced until finally each breath is expelled against only ambient pressure and the patient is fully weaned from the ventilator.
Problems arise upon transitioning from the inspiration phase to the expiration phase as the "command" pressure maintained by the exhalation valve is abruptly lowered from the desired inspiration pressure to the desired PEEP, and the patient begins to exhale. In a conventional ventilator system, the actual system pressure initially drops off precipitously to oscillate about the desired PEEP until ultimately equilibrating at a further reduced pressure. Such oscillation or "ringing" occurs at the natural frequency intrinsic to the particular system. The compressibility and volume of the respiratory gas, the flexibility and resiliency of the ventilator system's componentry and associated plumbing in contact with the gas as well as the patient's own physical constitution are all factors that influence the frequency and amplitude of this oscillation. Depending on the characteristics of a particular system, the amplitudes of oscillation can be substantial and the oscillations can continue for a significant portion of the expiration phase.
It is most desirable to minimize the described pressure oscillations and preferable to eliminate them altogether. Oscillation troughs below the desired PEEP level, albeit of short duration may have an adverse physiological effect due to the under pressurization of the alveoli structure. Additionally, such periods of reduced pressure may be misinterpreted by the ventilator system as an attempt by the patient to initiate a breath and may thereby auto-trigger a premature inspiration phase. Pressure peaks above the desired PEEP level, albeit of similarly short duration, require the patient to labor against excessive pneumatic pressures in an effort exhale. Additionally, the pressure excursions above the desired PEEP level cause the exhalation valve to open in an effort to maintain the desired PEEP which in doing so allows an excessive volume of gas to escape resulting in a ultimately lower than desired PEEP upon equilibration. The reduced PEEP is again undesirable for the physiological reasons set forth above as well as possibly causing the initiation of a premature auto-triggering of the inspiration phase.
Instability during inspiration is similarly undesirable. Fluctuations above command pressure may be misinterpreted as an attempt by the patient to exhale and could thereby auto-trigger a premature exhalation phase. Excursions below command pressures are indicative of a less than desired rate of air delivery into the lungs.
Previous attempts to control these oscillations within the ventilation system have focused on controlling the exhalation valve and have included efforts to reduce the lag time inherent in the operation of the valve itself as well as the damping of the valve's movements. Systems have been proposed wherein valve movement is damped at a constant rate such that the damping force is a direct function of a single variable such as system pressure, valve velocity or gas flow rate.
Although prior art efforts have reduced somewhat the described undesired pressure oscillations in the ventilator system, further reduction is desirable. Ideally, ventilator system pressure should closely follow, without significant deviation, the command pressure curve at all times, especially the step profile linking the relatively elevated plateau during inspiration to the lower PEEP plateau.